In this study it has been shown that increased diffusional resistances caused by salt stress may be fully overcome by exposing attached leaves to very low [ 06), that would implicate a reduced rate of RuBP regeneration, and, thus, a metabolic impairment. However, the analysis of the A -C i curves made on pre-conditioned leaves, showed that the estimates of the photosynthetic capacity parameters were much higher than in the standard A -C i responses. Moreover, these values were similar in magnitude to the average values reported by Wullschleger ( Journal of Experimental Botany 44, 907-920, 1993) in a survey of 109 C 3 species. These findings clearly indicates that: (1) salt stress did affect g s and g m but not the biochemical capacity to assimilate CO 2 and therefore, in these conditions, the sum of the diffusional resistances set the limit to photosynthesis rates; (2) there was a linear relationship ( r 2 = = = = 0.68) between g m and g s , and, thus, changes of g m can be as fast as those of g s ; (3) the estimates of photosynthetic capacity based on A -C i curves made without removing diffusional limitations are artificially low and lead to incorrect interpretations of the actual limitations of photosynthesis; and (4) the analysis of the photosynthetic properties in terms of stomatal and non-stomatal limitations should be replaced by the analysis of diffusional and non-diffusional limitations of photosynthesis. Finally, the C 3 photosynthesis model parameterization using in vitro -measured and in vivomeasured kinetics parameters was compared. Applying the in vivo -measured Rubisco kinetics parameters resulted in a better parameterization of the photosynthesis model.

Summary• The mechanism uncoupling isoprene emission and photosynthesis under drought was investigated in Populus alba saplings.• Isoprene emission, incorporation of 13 C into the isoprene molecule, isoprene synthase (ISPS) activity, concentration and gene expression, and photosynthesis were measured as a function of the fraction of transpirable soil water (FTSW) and in plants recovering from drought.• Photosynthesis sharply declined below FTSW 30 (a FTSW of 30%) and its inhibition was not caused by metabolic factors. A decline in isoprene emission was only evident towards the FTSW endpoint. 13 C incorporation into isoprene was lower when photosynthesis was constrained by drought. ISPS activity was inhibited by mild drought, while ISPS gene expression and concentration declined in concert with isoprene emission at the FTSW endpoint. Following rewatering, isoprene emission was higher than, and photosynthesis was similar to, prestress values. ISPS activity and concentration, and 13 C incorporation into isoprene, also rapidly recovered to prestress values, while ISPS gene expression remained low in rewatered plants.• Our experiment revealed a larger contribution of alternative carbon sources to isoprene emission only when photosynthesis was dramatically constrained by drought. Isoprene emission was likely controlled at the posttranscriptional level under severe drought.

A key objective for sustainable agriculture and forestry is to breed plants with both high carbon gain and water-use efficiency (WUE). At the level of leaf physiology, this implies increasing net photosynthesis (A N) relative to stomatal conductance (g s). Here, we review evidence for CO2 diffusional constraints on photosynthesis and WUE. Analyzing past observations for an extensive pool of crop and wild plant species that vary widely in mesophyll conductance to CO2 (g m), g s, and foliage A N, it was shown that both g s and g m limit A N, although the relative importance of each of the two conductances depends on species and conditions. Based on Fick's law of diffusion, intrinsic WUE (the ratio A N/g s) should correlate on the ratio g m/g s, and not g m itself. Such a correlation is indeed often observed in the data. However, since besides diffusion A N also depends on photosynthetic capacity (i.e., V c,max), this relationship is not always sustained. It was shown that only in a very few cases, genotype selection has resulted in simultaneous increases of both A N and WUE. In fact, such a response has never been observed in genetically modified plants specifically engineered for either reduced g s or enhanced g m. Although increasing g m alone would result in increasing photosynthesis, and potentially increasing WUE, in practice, higher WUE seems to be only achieved when there are no parallel changes in g s. We conclude that for simultaneous improvement of A N and WUE, genetic manipulation of g m should avoid parallel changes in g s, and we suggest that the appropriate trait for selection for enhanced WUE is increased g m/g s.

Olive ( Olea europea L) is one of the most valuable and widespread fruit trees in the Mediterranean area. To breed olive for resistance to salinity, an environmental constraint typical of the Mediterranean, is an important goal. The photosynthetic limitations associated with salt stress caused by irrigation with saline (200 m M ) water were assessed with simultaneous gas-exchange and fluorescence field measurements in six olive cultivars. Cultivars were found to possess inherently different photosynthesis when non-stressed. When exposed to salt stress, cultivars with inherently high photosynthesis showed the highest photosynthetic reductions. There was no relationship between salt accumulation and photosynthesis reduction in either young or old leaves. Thus photosynthetic sensitivity to salt did not depend on salt exclusion or compartmentalization in the old leaves of the olive cultivars investigated. Salt reduced the photochemical efficiency, but this reduction was also not associated with photosynthesis reduction. Salt caused a reduction of stomatal and mesophyll conductance, especially in cultivars with inherently high photosynthesis. Mesophyll conductance was generally strongly associated with photosynthesis, but not in salt-stressed leaves with a mesophyll conductance higher than 50 mmol m ----2 s ----1. The combined reduction of stomatal and mesophyll conductances in saltstressed leaves increased the CO 2 draw-down between ambient air and the chloroplasts. The CO 2 draw-down was strongly associated with photosynthesis reduction of saltstressed leaves but also with the variable photosynthesis of controls. The relationship between photosynthesis and CO 2 draw-down remained unchanged in most of the cultivars, suggesting no or small changes in Rubisco activity of saltstressed leaves. The present results indicate that the low chloroplast CO 2 concentration set by both low stomatal and mesophyll conductances were the main limitations of photosynthesis in salt-stressed olive as well as in cultivars with inherently low photosynthesis. It is consequently suggested that, independently of the apparent sensitivity of photosynthesis to salt, this effect may be relieved if conductances to CO 2 diffusion are restored.